Shapes in the Plane
Primary 4 · Mathematics
Shapes in the Plane
Lesson 1 of 5
25 min
Let's begin
On the way to school, Umutoni sees a road sign shaped like a triangle, and the pointed roof of a new house in her umudugudu. "Both are triangles," she says, "but they don't look the same at all."
Triangles come in families. Today you will sort them by the lengths of their sides.
A triangle is a closed shape with 3 straight sides, 3 corners (called vertices) and 3 angles.
Mathematicians draw small tick marks on sides that are equal. Sides with the same number of ticks have the same length. No ticks means no side is equal to another.
Measure each side with your ruler, then count how many sides match.
Example: a triangle has sides 6 cm, 9 cm and 6 cm. Two sides are equal, so it is isosceles. Its perimeter is 6 + 9 + 6 = 21 cm.
Example: a triangle has sides 4 cm, 5 cm and 7 cm. No sides match, so it is scalene.
Tip
"Equi" means equal. Equilateral: every side equal. Isosceles: two sides are twins. Scalene: all sides different, like three children of different heights.
Work each one in your exercise book first. Then tap Show answer to check.
A triangle has sides 8 cm, 8 cm and 8 cm. Name it.
Answer: Equilateral. All three sides are equal.
A triangle has sides 5 cm, 7 cm and 9 cm. Name it.
Answer: Scalene. No two sides are equal.
A triangle has sides 10 cm, 6 cm and 10 cm. Name it.
Answer: Isosceles. Two sides are equal.
An equilateral triangle has one side of 7 cm. What is its perimeter?
Answer: 21 cm, because 7 + 7 + 7 = 21.
Two sides of a triangle have 2 ticks each and the third side has none. What kind of triangle is it?
Answer: Isosceles.
What you learned today
Sources and credits
The stories, Rwandan examples, diagrams and practice questions were written by Ganzaa.
Quick check
A triangle has sides 5 cm, 8 cm and 5 cm. What is it?
